<html>

<body id="myid" class="mystyle">

    <script type="text/javascript">
        x = document.getElementsByTagName('body')[0];
        console.log(x);
        
        document.write("Body CSS class: " + x.className);
        document.write("<br />");
        document.write("An alternate way: ");
        document.write(document.getElementById('myid').className);
    </script>

</body>

</html>